This application claims the benefit of and incorporates by reference co-pending Israeli application No. 143,106 filed May 13, 2001.
The present invention relates to an improved process for the preparation of diphenylmethylthioacetamide (I), a key intermediate in the synthetic pathway for Modafinil.
Modafinil is defined as a CNS stimulant and is marketed under the trade name xe2x80x9cProvigilxe2x80x9d, for the treatment of narcolepsia.
Modafinil was introduced by Lafon. Its preparation, use and pharmacological properties were described in U.S. Pat. No. 4,177,290.
U.S. Pat. No. 4,177,290 describes the synthetic pathways for the preparation of Modafinil (III). Both of the pathways described therein use diphenylmethylthioacetic acid (II) as starting material. 
One synthetic method is based on the reaction of diphenylmethylthioacetyl chloride (obtained from II and thionyl chloride) with ammonia, followed by oxidizing the thioamide formed with hydrogen peroxide. Total yield, as reported, is 63%
(Scheme I). 
A different synthetic pathway described in the same patent involves the oxidation of II as first stage, followed by esterification with dimethylsulfate and amidation with gaseous ammonia. The total yield is 41% (Scheme 2). 
Compound II is prepared by the reaction of diphenylmethanol with thiourea in the presence of HBr followed by basic hydrolysis and reaction with chloroacetic acid (Scheme 3). 
Both synthetic methods involve four chemical steps (starting from diphenylmethanol) and the yields, as reported, are moderate.
Furthermore, both pathways involve toxic and dangerous reagents such as dimethylsulfate, ammonia and thionyl chloride.
In order to improve the synthetic method, according to the present invention there is now reacted the isothiouronium salt (IV) (obtained from the reaction between diphenylmethanol and thiourea) with haloacetamide.
According to the present invention it has now been found that haloacetamide reacts with the isothiouronium salt (IV) in protic solvents (like alcohols or water) at temperatures ranging from 0-100xc2x0 C. in the presence of a strong base, such as alkali metal hydroxide.
Preferably, chloracetamide is reacted with the said isothiouronium salt in water at 60-70xc2x0 C. using sodium hydroxide as base.
When the reaction is carried out under the above-mentioned conditions diphenylmethylthioacetamide is obtained in 95% yield from diphenylmethanol. Thus according to the present invention there is now provided a process for the preparation of diphenylmethylthioacetamide (I) as described in Scheme 4 comprising reacting of the isothiouronium salt or its corresponding base of the formula IV with an acetamide of the formula XCH2CONH2, wherein X represents a halogen, M represents an alkali metal and A represents an anion, in a protic medium at a temperature of less than 100xc2x0 C. 
The said method comprises only three chemical steps. It does not involve toxic or corrosive reagents. Very pure Modafinil is obtained at the end of the synthetic pathway at 67% yield (starting from diphenylmethanol).
Thus, diphenylmethanol is reacted with thiourea in the presence of hydrobromic acid in aqueous medium under reflux and then cooled. The isothiouronium salt (IV) formed is filtered and reacted with chloroacetamide under aqueous base conditions at 60-70xc2x0 C. The solution is cooled and the diphenylmethylthioacetamide formed is filtered and collected.
All the physical properties and the NMR and IR spectra are in perfect agreement with the proposed structure.
The following examples will serve as further illustration and clarification of the present invention.
While the invention will now be described in connection with certain preferred embodiments in the following examples so that aspects thereof may be more fully understood and appreciated, it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the scope of the invention as defined by the appended claims. Thus, the following examples which include preferred embodiments will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purposes of illustrative discussion of preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of formulation procedures as well as of the principles and conceptual aspects of the invention.